This grant addresses aspects of the general question: how do neural and hormonal pathways work together to establish regulation of refraction and growth of both eyes during early postnatal development? These pathways are important in coordinating development of eye movements, eye size, refraction and emmetropization. As such, their investigation is of singular importance in understanding the genesis of refractive error, strabismus, and amblyopia in humans. This study is particularly focused on the regulation of growth of the anterior segment of the eye. Using the chick model we will 1a) determine if the signal responsible for protection from constant light effects (flattened cornea, shallow anterior chamber & hyperopia) is a periodic increase in melatonin concentration, and investigate the causal chain linking illumination and melatonin concentrations; 1b) determine whether the elongation of the vitreous chamber in constant light results from emmetropization growth, induced by the hyperopia that is caused by the flattening of the cornea; 1c) investigate the role of autonomic effects on the growth of the anterior segment; 2) investigate the light-dependent growth influences of one eye on another by various manipulations including optic nerve section, selective retinal cell destruction by drugs, and the generation of unilaterally micro-ophthalmic chicks. 3) investigate the mode of coupling of the light dark cycle to the circadian melatonin rhythm with the aid of mathematical models. Behavioral, surgical, biochemical, histological, and mathematical methods will be employed to answer these questions.